Devices for generating plasma from which ions or electrons are extracted are widely used in industry for effecting surface treatments (ion etching, cleaning, material deposition, ion implantation, etc.), while in the space sector they are finding widespread application as ion propulsors, as satellite charge neutralizers or as satellite/surrounding plasma contactors.
An ion generating device of the conventional type is schematically shown by way of example in FIG. 1. It comprises an ionization chamber 1 (where the plasma is generated) and an extraction system 2 which extracts the charged particles generated inside the chamber. A substance in the form of a gas or vapor is introduced into the ionization chamber, via the supply means 4, from which substance (using various methods known per se) the positive ions of the desired chemical species and free electrons are obtained. The ions are extracted from the ionization chamber, focused and accelerated towards the target by the extraction system 2. The device denoted by 3 represents a source of electrons for possible neutralization of the beam, where this is required, such as for example in the space sector, to prevent the satellite on which the device is mounted from becoming negatively charged. The electron source is not required, however, in cases where the ion generator is used for discharging a positively charged satellite. Ionization of the introduced gas produces, inside the ionization chamber 1, a plasma containing positive ions which are useful for forming the ion bee and free electrons which, when suitably further accelerated, are able to ionize other neutral atoms, thus producing further ions and free electrons. This process is sustained by a continuous supply both of neutral atoms (gases), in exchange for the extracted ions, and of electrical energy for accelerating the free electrons. The electrical energy is supplied via appropriate power supply units 5 depending on the various methods used, the most common of which are direct current discharging and discharging obtained by accelerating the electrons present using radiofrequency or microwave fields.
The process for triggering discharging is based, initially, on the transfer of energy (via radiofrequency or constant electric fields) to the free electrons present in the non-ionized gas. These electrons, usually present in very small quantities, are produced as a result of background radiation, cosmic rays, etc. The free electrons, by absorbing energy from the electric fields suitably supplied by the appropriate power supply units, trigger the process of multiplication of electrons and ions in the gas. Sometimes (in particular in devices which use radiofrequency) the quantity of free electrons present is not sufficient to trigger discharging. Delays may therefore be observed between the start of the action of the electric fields and stabilization of the plasma inside the chamber, or else electric fields of particularly high amplitude are required.
In many devices used for space applications--in ion propulsion or for neutralizing the charge of satellites--methods involving preionization of the gas with arc discharges are used or it is attempted to increase the number of free electrons inside the discharge chamber by attracting them from an external source (hollow cathode, heated filaments, etc.). These elements, when not required for other reasons, increase the complexity of the system and reduce its reliability since they are susceptible to malfunctions. This constitutes a notable drawback.